Cell Structure Of Foams Research Articles

Metal Ions Fortified Tannin-Furanic Rigid Foam: The Impact on the Uniformity and Mechanical Performance

Tannin-furanic foams with excellent properties have attracted increasing interest due to their advantages such as easy preparation, light weight, and thermal insulation. However, unsatisfactory mechanical strength has limited the expansion of their applications. Herein, three different metal ions (Cu2+, Fe3+, and Zn2+) were chosen to enhance the properties of tannin-furanic foam prepared by mechanical stirring provoked a foaming approach. The positive effects originating from the complexation are attributed to the associated connection between tannin molecules and metal ions. The results indicated that the apparent performance was improved, resulting in even foam cell structures. The apparent densities for the tannin-furanic foam modified with metal ions were located in the range of 36.57–47.84 kg/m3, showing the feature of lightweight material. The enhanced mechanical strength was verified by the compression strength (0.097–0.163 MPa) and pulverization ratio (7.57–11.01%) of the modified foams, which increased by 56–163% and decreased by 61–73%, respectively, in comparison with tannin-furanic foam without the metal ions. Additionally, the thermal conductivity of the modified tannin-furanic foams was in the range of 0.0443 to 0.0552 W/m·K. This indicates that they inherited the excellent thermal insulation typically associated with tannin-based foams. Interestingly, higher mechanical performance was obtained by comparison with other bio-sourced foams even with similar densities. In summary, by introducing only a small amount of metal ions, the foam performance was greatly improved, with a moderate cost increase, which reflects a good development prospect.

Investigation of Damping Properties of Natural Fiber-Reinforced Composites at Various Impact Energy Levels.

Natural fiber-reinforced composites are composite materials composed of natural fibers, such as plant fibers and synthetic biopolymers. These environmentally friendly composites are biodegradable, renewable, cheap, lightweight, and low-density, attracting attention as eco-friendly alternatives to synthetic fiber-reinforced composites. In this study, natural fiber-reinforced polymer foam core layered composites were produced for the automotive industry. Fabrics woven from goat wool were used as the natural fiber. Polymer foam with expanded polystyrene (EPS) and extruded polystyrene (XPS) structures was used as the core material. During production, fibers were bonded to the upper and lower layers of the core structures using resin. The hand lay-up method was used in production. After resin application, the samples were cured under a heated press for 2 h. After the production was completed, the material was cut according to the standards (10-20-30 Joule), and impact and bending tests were conducted at three different energy levels. The experiments revealed that at 10 J, the material exhibited rebound; at 20 J, it showed resistance to stabbing; and at 30 J, it experienced penetration. While EPS foam demonstrated higher impact resistance in the 10 J test, it was found that XPS foam exhibited better impact resistance and absorption capabilities in the 20 J and 30 J tests. Due to the open and semi-closed cell structure of EPS foams and the closed cell structure of XPS foams, it has been concluded that XPS foams exhibit higher impact resistance and better energy absorption properties.

Pressure Distribution of Polyurethane Foam Based on Palm Oil Content

This study explores the effect of varying epoxidized palm oil (EPO) content on the mechanical properties and pressure distribution of polyurethane (PU) foam. Polyurethane foams were synthesized with EPO concentrations of 25%, 50%, and 75%, and their morphologies, densities, hardness, compressive strength, and pressure distribution capabilities were analyzed. The results showed that increasing EPO content significantly affects the foam's cell structure, leading to variations in density and mechanical properties. PU foam with 50% EPO exhibited a more open-cell structure, resulting in lower density and hardness but compromised compressive strength. Conversely, foam with 75% EPO content demonstrated improved pressure distribution despite increased density and nonuniform cell sizes. These findings highlight the potential of using palm oil-based polyols to develop sustainable PU foams, balancing flexibility and mechanical strength for applications in cushioning and pressure relief materials. The study underscores the importance of optimizing EPO content to achieve desirable foam properties for specific applications.

Shape Memory Polymer Foam Based on Nanofibrillar Composites of Polylactide/Polyamide.

This paper presents the novel development of a shape memory polymer foam based on polymer-polymer nanocomposites. Herein, polylactide (PLA)/biosourced polyamide (PA) foams are fabricated by in situ fibrillation of polymer blends and a subsequent supercritical CO2 foaming technique. In this system, PLA serves as a shape memory polymer to endow this foam with a shape memory effect (SME), and in situ generated PA nanofibers are employed to reinforce the PLA cell walls and provide an additional permanent phase. A concentration of PA, 5 wt.%, was chosen to form an entangled nanofibrillar network. Foams of PLA/PA nanoblends with the same content of constituents were fabricated to reveal the effect of minor phase morphology on the cell structure and shape memory behavior of polymer foams. Profiting from the reinforcing effect of PA nanofibers, the PLA/PA nanocomposite foam exhibits smaller foam cells, a narrower cell size distribution and a comparable cell concentration than the PLA/PA nanoblend foam. In addition, PA nanofibers, unlike PA nanodroplets, favor the shape fixation ratio and recovery ratio and shorten the shape recovery time.

Study on foaming behavior of block copolymer/inorganic particles co‐construction of toughened epoxy resin foams

AbstractLow toughness and brittleness limit its application range of thermosetting epoxy resin (EP) based foams. In current research, inorganic particles are commonly utilized as heterogeneous nucleating agents to enhance foaming quality and EP toughness. However, the increasing inorganic particle content usually leads to particle agglomeration, which negatively impacts foaming quality and reduces EP performance. Block copolymers, which appear as liquid to effectively mitigate agglomeration, are employed as toughening materials for EP. In this study, a small number of modified calcium silicate particles (MCS) were used as anchor points to induce the distribution of block copolymers (P‐F68) to improve EP toughness and effectively enhance cell nucleation, thus, EP/MCS/P‐F68 foams performance was improved. The research results showed that EP foams obtained excellent cell structure and very small foam density (0.256 g/cm3) at the P‐F68 content of 10 wt%. The continuous increase of P‐F68 content would lead to the thickening of cell wall and the increase of cell size. Simultaneously, the core‐shell structure formed by P‐F68 in EP effectively enhanced the toughness of EP foams and mitigated its brittleness, offering crucial theoretical guidance for toughening and regulating the cell structure of EP foams.

The shear performance of uniaxially thermoformed auxetic polymer foams

We study the shear behavior of pristine and uniaxially thermoformed auxetic foams using three different shear testing techniques and finite element simulations. A novel simple shear test rig with sliding lateral supports was developed and compared to fixed simple shear and diagonal pure shear rigs. The three rigs yield similar results at shear strains under 8 %, but the fixed simple shear rig shows a significant rise in shear modulus and stiffness at higher strains. In contrast, the sliding simple shear and diagonal pure shear rigs exhibit similar linear results up to 20 % shear strain. The thermoformed auxetic foam displays strong orthotropy, with a shear modulus of 35 kPa along the thermoforming direction and 0.1 MPa in the transverse directions. The pristine foam shows good isotropy under shear, with a similar 50 kPa shear modulus in all directions. The auxetic foam expands slightly along the thermoforming direction at high strains, while the pristine foam and auxetic foam in other directions show noticeable shrinkage. Finite element models based on 3D μ-CT scans, using a new boundary coupling element to apply periodic boundary conditions, closely matched the experimental results. The simulations revealed the deformation mechanisms within the foam cell structures that contributed to the observed orthotropic shear behavior.

Hierarchical microstructure changes of oriented poly(vinylidene fluoride) foam via supercritical carbon dioxide with increased piezoelectric performance

Light weight poly (vinylidene fluoride) (PVDF) has attracted wide attention for energy supply. In this work, oriented PVDF foams were prepared using supercritical carbon dioxide (CO2) physical foaming associated with a constraint mold. The corresponding micro-nanostructures and electrical performances were researched comprehensively. It reveals that the cell structure and molecular chains of PVDF foams are oriented due to the limitation of gas diffusion paths. Because of the realization of uniaxial stretching during cell growth, the β phase content is increased. Besides, after foaming, the regularity of the chain segments in amorphous region is improved, which is caused by the decrease in the quantity of free-volume cavities. The 1D-149 °C oriented PVDF foam exhibits a high voltage output (8 V), due to the ultralow dielectric constant (1.8) and high compressive strength. This study provides a theoretical foundation for the design of oriented PVDF foams, showing the potential of energy collector.

Evaluating how citrus fiber affects the cell structure and functions of starch-based foam

This work used a proper content of citrus fiber (10 wt% ∼ 30 wt%, dry starch base) to improve the cell structures and functions of starch foam. The insoluble fiber promoted cell nucleation in the starch melt, whereas the soluble pectin improved the starch matrix's strength by forming ester bonding with starches as confirmed by FTIR results. This effectively modified the starch foaming process and endowed the obtained foams with an increased cell density (from 83.37 to 123.44 cell/cm3×104) and a reduced foam density (from 0.457 to 0.142 g/cm3). The foam containing 20 wt% citrus fiber exhibited the highest expansion ratio of 5.20. Due to the improved structures and the tortuous path effect of cellulose crystals, the prepared foams significantly presented a strengthened compression performance and improved moisture resistance. Meanwhile, the fiber-loaded foams displayed a high free radical-scavenging activity (60.30 % ∼ 85.75 %), showing good antioxidant activity. It is therefore believed that this study could provide a facile and green way for producing starch-based foams with desirable functions.

Study on the Cross-Scale Effects of Microscopic Interactions and Mechanical Properties of Rigid Polyurethane Foam Driven by Negative-Temperature Environments.

In order to investigate the cross-scale effects of the interaction between the hard and soft segments of stiff polyurethane foam on the material's mesoscopic pore structure and macroscopic compression characteristics in various negative-temperature environments, this paper used molecular dynamics to calculate the interaction differences between hard and soft segments in different negative-temperature environments. The effects of various negative-temperature settings on the cell structure of stiff polyurethane foam were investigated using scanning electron microscopy and Image J software. Finally, macro experiments were used to determine the influence of a negative-temperature environment on the characteristics of stiff polyurethane foam (such as compressibility). The molecular simulation calculation results show that in a negative-temperature environment, decreasing temperature gradually increases the interaction between hard segment molecules and soft segment molecules, resulting in an increase in the molecules' modulus and cohesive energy density. The scanning electron microscope results reveal that a negative-temperature environment gradually increases the pore diameter of stiff polyurethane foam. The compression experiment findings demonstrate that, for the same service duration, the compressive strength in the -20 °C environment is 27.53% higher than that in the 0 °C environment. The study's findings reveal a microscopic mechanism for the following receiving alterations and toughness enhancement of rigid polyurethane foam throughout service in negative-temperature conditions.

Highly Electroconductive Metal-Polymer Hybrid Foams Based on Silver Nanowires: Manufacturing and Characterization.

Due to their electroconductive properties, flexible open-cell polyurethane foam/silver nanowire (PUF/AgNW) structures can provide an alternative for the construction of cheap pressure transducers with limited lifetimes or used as filter media for air conditioning units, presenting bactericidal and antifungal properties. In this paper, highly electroconductive metal-polymer hybrid foams (MPHFs) based on AgNWs were manufactured and characterized. The electrical resistance of MPHFs with various degrees of AgNW coating was measured during repeated compression. For low degrees of AgNW coating, the decrease in electrical resistance during compression occurs in steps and is not reproducible with repeated compression cycles due to the reduced number of electroconductive zones involved in obtaining electrical conductivity. For high AgNW coating degrees, the decrease in resistance is quasi-linear and reproducible after the first compression cycle. However, after compression, cracks appear in the foam cell structure, which increases the electrical resistance and decreases the mechanical strength. It can be considered that PUFs coated with AgNWs have a compression memory effect and can be used as cheap solutions in industrial processes in which high precision is not required, such as exceeding a maximum admissible load or as ohmic seals for product security.

The effect of the ceramic foam cell structure on the combustion performance of porous media burner: experimental study

To investigate the effect of the ceramic foam cell structure on the combustion characteristics, a porous media burner(PMB) composed of foamed ceramic plates with Kelvin and WP cell models as structural features was designed and developed. The combustion chamber temperature, lean combustion limit, radiation efficiency and flue gas emission characteristics of the burner were analysed in the combustion of methane. The results show that with the same pore density, skeleton diameter and operating parameters, the combustion chamber temperature and radiation efficiency of the WP model are higher than those of the Kelvin model, while the CO and NOx emissions of WP model have an opposite tendency. When the ceramic foams are composed by WP cell model, under the same pore density and operating parameters, the burner has better combustion performance when the skeleton is thin. The combustion chamber temperature and radiation efficiency are higher, CO and NOx emissions are lower, and the lean combustion limit equivalent ratio is lower.

EMI performance within the conductive composite foams engineered with bimodal and uniform cell structures

The electromagnetic interference shielding (EMI) performance of conductive polymer composite foam is significantly influenced by the foam cell structure. In this work, the impacts of cell structure, cell size and foaming degree on the electric conductivity (EC) and EMI shielding of the foams were investigated. The results showed that the specific EMI shielding performance of the composites was improved by a factor of 2–3 after foaming, and the bimodal cell foams featured higher EC (∼1.30 S/m) but lower EMI shielding performance (∼10.52 dB) compared to the uniform cell foams, owning to their segregated conductive network. Furthermore, the EC and EMI of the bimodal cell foams decreased with the increase of foaming degree, while for the uniform cell foams, maximum specific EMI was obtained at a void fraction of ∼60%. These findings contribute to the design and fabrication of new polymer composite foams with high EMI shielding capability.

EFFECTS OF FOAMING PARAMETERS ON PHYSICAL PROPERTIES OF MICROWAVE-VULCANIZED NATURAL RUBBER LATEX FOAMS (IVCST2021)

This work reported the use of microwave vulcanization coupled with Dunlop process to prepared natural rubber latex foam (NRLF). Optimization of microwave heating for curing step was firstly carried out using a commercially available domestic microwave oven. The processing window and optimum condition for making NRLF were obtained. The optimum power and time of the irradiation were found at 600 W and 6 min, respectively. Using the optimum microwave vulcanization, effects of stirring time (2, 4, 6, 8, and 10 min) and speed (650, 950, 1250, 1550, and 1850 rpm) of foaming step on cell structure, bulk density, compressive properties, and hardness of NRLFs were investigated. Generally, the increases in the stirring time and speed in foaming step both gave rise to the volume of the air incorporated into the latex compound. This resulted in the increases in the bulk density and the number of cell but, the decrease in foam cell size. The stirring speed in a range of 950 to 1550 rpm was successfully used for preparing the NRLF. Too low and too high foaming speeds caused unstable latex compounds and failed to produce NRLF. The foam cell structure changed dramatically as the foaming speed increased. The foam mechanical properties, both compressive strength and hardness, were found to be linearly related to the foam bulk density.

Study on preparation and electromagnetic shielding properties of GNs/NaCl/SCF composite carbon foam

Herein, the graphenes/sodium chloride/sucrose composite carbon foam (GNs/NaCl/SCF) was prepared by high pressure pyrolysis and chemical vapor deposition (CVD) It was found that NaCl micron particles played the role of nucleating agent in the foaming process of sucrose based carbon foam, which could effectively tailoring the pore structure of composite carbon foam. When the thickness of GNs/NaCl/SCF is only 3 mm, the total shielding efficiency (SET) of composite carbon foam in the X-band range is up to 49.25 dB, and the absorption loss (SEA) is much greater than the reflection loss (SER). The SET of GNs/NaCl/SCF was 2.8 times higher than that of pure sucrose based carbon foam (SCF). This is due to the unique structure of the composite carbon foam. First, the rich 3D continuous pore cell structure of carbon foam improves the impedance matching degree of electromagnetic wave, so that the electromagnetic wave “trapped” in the pore cell, forming multiple reflection and scattering. Second, under the catalysis of elemental copper, the graphene grown in situ on the pore cell of the carbon foam formed a conductive network, which significantly improved the conductivity of the composite carbon foam. Third, the composite carbon foam has abundant heterogeneous interfaces (such as Cu-GNs, Cu-SCF, GNs-SCF, etc.), which act as the center of polarization of induce dipole polarization to further enhance the electromagnetic shielding performance. Sequentially, the synergistic effect of the above three aspects leads to the significant improvement of the electromagnetic shielding performance of composite carbon foam.

Effect of Selected Bio-Components on the Cell Structure and Properties of Rigid Polyurethane Foams.

New rigid polyurethane foams (RPURFs) modified with two types of bio-polyols based on rapeseed oil were elaborated and characterized. The effect of the bio-polyols with different functionality, synthesized by the epoxidation and oxirane ring-opening method, on the cell structure and selected properties of modified foams was evaluated. As oxirane ring-opening agents, 1-hexanol and 1.6-hexanediol were used to obtain bio-polyols with different functionality and hydroxyl numbers. Bio-polyols in different ratios were used to modify the polyurethane (PUR) composition, replacing 40 wt.% petrochemical polyol. The mass ratio of the used bio-polyols (1:0, 3:1, 1:1, 1:3, 0:1) affected the course of the foaming process of the PUR composition as well as the cellular structure and the physical and mechanical properties of the obtained foams. In general, the modification of the reference PUR system with the applied bio-polyols improved the cellular structure of the foam, reducing the size of the cells. Replacing the petrochemical polyol with the bio-polyols did not cause major differences in the apparent density (40-43 kg/m3), closed-cell content (87-89%), thermal conductivity (25-26 mW⋅(m⋅K)-1), brittleness (4.7-7.5%), or dimensional stability (<0.7%) of RPURFs. The compressive strength at 10% deformation was in the range of 190-260 and 120-190 kPa, respectively, for directions parallel and perpendicular to the direction of foam growth. DMA analysis confirmed that an increase in the bio-polyol of low functionality in the bio-polyol mixture reduced the compressive strength of the modified foams.

Effect of Silica Content on Cellular Structural and Hygroscopicity of Modified Cassava Starch Foam

AbstractThe growth in polymer production is influenced by the high demand from the packaging sector. Expanded polystyrene (EPS) is one of these materials that is widely used in disposable packaging, however, this generates waste that occupies a large volume and is difficult to degrade. Starch, a naturally occurring and biodegradable polysaccharide, may be an alternative in the search for a renewable source material. However, starch foams have some limitations related to their hygroscopicity. The objective of this study is to analyze the influence of silica content (1%, 2%, and 3%) on the cell structure and surface hygroscopicity of cassava starch foams. The foams containing starch, glycerol, and water are produced by thermo‐compression molding. Results show that, at lower contents, silica acts predominantly as a nucleating agent, generating smaller cells in the foam. At the higher silica content, it plays a more efficient role in reducing the affinity of the starch for water. The formulation with 2% silica shows the highest density (0.232 g cm−3), impact strength (2067 J m−2), and contact angle (84.8°).

Enhanced sensing performance of EVA/LDPE/MWCNT piezoresistive foam sensor for long-term pressure monitoring

Conductive polymer composites (CPCs) with piezoresistive properties are smart detecting systems with high sensitivity, fast response, and low energy consumption. To enhance their sensitivity and cope with their high filler content requirements, solid CPCs are normally transformed into blends and foams, which reduces the elastic modulus, enabling larger deformations, wider detection range, and increased sensitivity. However, providing reversible and durable sensing properties remains challenging as it depends on the phase morphology, foam cell structure, and particle dispersion. To this end, we use multi-walled carbon nanotube (MWCNT) as a conductive filler with a high aspect ratio and develop CNT-filled ethylene vinyl acetate/low-density polyethylene (EVA/LDPE) blend foams. We fix the foam structure using chemical foaming and crosslinking agents in a compression molding process. To optimize the piezoresistive behavior, we follow a step-wise tuning approach, studying the blending morphology, the foaming attributes, the particle dispersion and affinity, thereby selecting the most promising formulations for further investigations. The least mechanical and electrical hysteresis is obtained at minimum foam density, which is obtained at a compromised crystallinity and crosslinking density. The higher affinity of MWCNT to EVA and the faster crosslinking of the EVA phase results in uniformly dispersed small cells at EVA-rich formulations and 4 phr MWCNT content. The final foam sensor demonstrates an impressive response and recovery times of less than 200 ms and remarkable durability thanks to the optimized cell morphology and mechanical properties. These findings expand the application of carbon-filled polyolefin foams in pressure monitoring, particularly in wearable sensors for strain and motion sensing.

Effect of crystalline transformation on supercritical CO2 foaming and cell morphology of isotactic polybutene-1

As a polycrystalline polymer, isotactic polybutene-1 (iPB-1) will form different crystalline structures when it crystallizes under different conditions. In this work, we found a simple method to manipulate the cell structure of iPB-1 foams by controlling the annealing time of crystalline form II at room temperature and employing supercritical CO2 foaming. As the content of crystalline form II increases, the storage modulus (G′) and complex viscosity (|η * |) increase, and the solubility of CO2 in iPB-1 increases. Crystalline form II has a wider foaming temperature window compared to crystalline form I and exhibits a more uniform cell structure at temperatures between 95 °C and 125 °C. The average cell diameters of crystalline form II and I are 33.9 µm and 11.2 µm at 115 °C and 15 MPa CO2, respectively. A small amount of crystalline form I acts as a heterogeneous nucleation agent during the foaming process, resulting in the formation of a “petal-like” cell structure. During the foaming saturation process, the melted portion of iPB-1 with unstable crystalline form II transforms into crystalline form I′ under high-pressure CO2, while the unmelted portion transforms into crystalline form I. As the pressure increases, the cell structure of iPB-1 undergoes a transition from “petal-like” to bimodal cell structure, ultimately achieving a more uniform structure. Moreover, increasing foaming pressure can also change the cell structure of the foamed material from closed-cell to open-cell. The occurrence of bimodal melting peaks in the foamed samples is beneficial for the adhesion of beads during the molding process.

Controllable cell structures of poly(ether-block-amide) foams via isothermal melt crystallization-foaming in supercritical CO2

A simple isothermal melt crystallization-foaming method was proposed to regulate the crystallinity (Xc) of PEBA to prepare poly(ether-block-amide) (PEBA) foams with uniform or bimodal cell structures by supercritical CO2. PEBA with different Xc values was obtained by controlling the isothermal crystallization temperature (Tiso) and time (tiso). The rheological and DSC results showed that the viscoelasticity and Xc of the samples increased with decreasing Tiso and increasing tiso. PEBA foams with Xc above 1.1% exhibited a distinct bimodal cell structure. The increased crystals acted as heterogeneous nucleation sites to increase the cell density from 6.8 × 106 cells/cm3 to 3.58 × 1010 cells/cm3. The size of large cells decreased from 206.4 µm to 48.0 µm, and the number ratio of small to large cells increased from 2.34 to 65 times. Finally, a mechanism to explain the cell structure variation in PEBA foams was proposed, providing a new strategy for regulating the cell structure of polymeric foams.

Preparation of clay-supported sodium benzoate and its effect on the rheology, crystallization and foaming of long chain branched polypropylene

The foaming of PP has encountered challenges because of its low melt strength and semi-crystalline characteristics. Introducing long chain branching (LCB) onto the PP backbone and the addition of particles as cell nucleating agent are the commonly used methods to improve the cell structure of PP foams. In this paper, clay-supported sodium benzoate (NaB-OMMT) was prepared from Na-montmorillonite (Na-MMT) via the partial ion exchange reaction with surfactant and subsequent reaction with benzoic acid. The as prepared NaB-OMMTs were then applied to prepare LCBPP/NaB-OMMT nanocomposites and their foams. The dispersion state of clay, crystallization behaviors and rheological properties of the nanocomposites, as well as the cell structure and compression properties of the foams were investigated. The results show that clay layers are mostly exfoliated in the nanocomposites with low NaB-OMMT content, while intercalated layers dominant with the content reaches 2% or higher. The melting temperature of the nanocomposites decreases and the crystallinity increases, while the spherulite size decreases compared to LCBPP. The melt viscosity and elasticity of nanocomposites increase significantly when the content of NaB-OMMT reaches 2% or higher. LCBPP/NaB-OMMT nanocomposites foams exhibit higher cell density, smaller cell size and its standard deviation. Therein, the foam with 1% of NaB-OMMT shows the maximum cell density and the minimum cell size. It is concluded that the cell structure of the foam is highly dependent on the content, dispersion state and structure of the cell nucleating agent. When NaB is supported on well dispersed and exfoliated clay layers, the cell nucleation efficiency is significantly enhanced. The compression properties of the foams depend on their cell structures and exhibit the similar variation trend with the latter with the content of NaB-OMMT increasing.